Hybrid vehicles, electric vehicles and other similar electric-powered vehicles that can employ an electric motor to travel employ electric power storage devices in the form of a lithium ion battery, a nickel hydrogen battery or a similar secondary battery, or an electric double layer capacitor of large capacitance or the like to supply the electric motor with electric power. How an electric power storage device is charged is indicated by an amount of state, which is generally indicated by a state of charge (SOC). A fully charged state is represented by an SOC=100% and an amount of charge of 0 is represented by an SOC=0% to represent the electric power storage device's state of charge. Hereinafter a state of charge will also simply be referred to as an “SOC”.
Japanese Patent Laying-open No. 2000-258513 discloses an SOC calculation method that can precisely calculate a secondary battery's SOC. The SOC calculation method corrects a predetermined resistance value that is previously provided for a battery in accordance with the battery's temperature to calculate the battery's internal resistance, which determines the battery's voltage-current characteristic, and in accordance therewith an open circuit voltage (hereinafter also referred to as “OCV”) is calculated, and an OCV vs. SOC correlation representing a correlation between OCV and SOC is used to calculate an SOC based on the calculated OCV.
However, the SOC calculation method of Japanese Patent Laying-Open No. 2000-258513 calculates an SOC repeatedly for a period which starts once the vehicle's power supply is turned on and which continues until it is turned off, including a period for which the vehicle is traveling. It is thus significantly affected by disturbance.
For example, a battery's voltage V is affected by internal resistance and in addition so called polarization voltage, which is varied by a history of charging/discharging frequently repeatedly, and thus affects precision of calculation of OCV. Note that it is well known that polarization can be resolved by charging/discharging to be significantly away from a range in which SOC is controlled.
Furthermore, while a vehicle is traveling, its surrounding environment also significantly varies, and the above publication's SOC calculation method is in this regard also significantly affected by disturbance and SOC may be estimated with poor precision.